ESSAYS ON BACTERIOLOGY. 53 



the liTing germs, but weakened, claiming tlius to 

 more closely imitate nature. This weakening or at- 

 tenuation of the virus is brought about in various 

 ways: by altering the culture soil, by heat, gases, vary- 

 . ing atmospheric pressure, drying, and by passing the 

 germs through the bodies of animals. Pasteur started 

 the line of work which has made him famous upon 

 the supposition that vaccinia Avas simply smallpox 

 modified by passing through the body of the cow. 

 Whether this is true or not, his theory of the use of 

 attenuated virus proved correct, and led to the aston- 

 ishing results in the diseases already mentioned. 

 Which line of work will best sustain its claims in the 

 end remains for the future to decide. Certainly thus 

 far the living virus seems to have the more to its 

 credit. 



Let us now look for a moment at several other in- 

 teresting facts, to which I shall refer but briefly be- 

 fore attempting to sum up the matter of our discus- 

 sion. 



First, as already indicated, the blood, or body 

 juices, of animals, naturally immune, may afford 

 more or less protection to susceptible animals against 

 a disease to which the one is immune. This fact may 

 open the way to some valuable results. 



Second, it may iiltimately be found that the appar- 

 ent absence, or slight tendency to self-limitation and 

 immunity in certain diseases, is. only apparent; that 

 these tendencies are present, but are overcome by an- 



